In many electronic electricity meters, communications with the meter microcomputer can be performed via an optical port or an option board connector. For example, in some known meters, an electrical connector is provided so that various option boards, such as a telephone modem communication board, may be electrically connected to the meter microcomputer. In addition, an optical port electrically coupled to the meter microcomputer and accessible to devices external to the meter may also be provided. Asynchronous serial data may be communicated to the meter microcomputer via the optical port and the option board connector. All the meter data is available in a predefined format (an ANSI defined protocol) on the communication channel which connects the port and the option board connector to the microcomputer.
To prevent theft of or tampering with the metering data, it is desirable to control option board access to the meter microcomputer via the communication channel. Preventing such access, however, should not inhibit communication with the meter microcomputer via the optical port.
It would be desirable to provide a data source identification unit, or circuit, which transmits a signal to the meter microcomputer indicative of the source of communication, i.e., either the optical port or the option board connector. It also would be desirable to provide such a circuit without significantly increasing the costs associated with fabrication of a meter.